Dynamin is a large, extended protein consisting of five domains. Recently, we determined the crystal structure of a human dynamin 1-derived minimal GTPase-GED (for GTPase effector domain) fusion protein. This fusion protein dimerizes in the presence of the transition state mimic GDP-aluminum fluoride. The structure revealed dynamin's catalytic machinery and explained how assembly-stimulated GTP hydrolysis is achieved through G domain dimerization. Particularly intriguing was the identification of a sodium ion in the active site, suggesting that dynamin uses a cation to compensate for the developing negative in the transition state and providing a rationale for the inability to previously implicate a more-usual arginine finger. More recently, we have determined the structure of the GTPase-GED fusion with a ground state GTP analog, GMP-PCP. The structure showed a surprising reorientation of a part of the molecule. We have been able to combine this information with cryo-EM structural information obtained of the helical tubes formed by GMP-PCP-bound dynamin (more specifically, a truncated form called delta-PRD). These data together suggest a specific mechanism by which dynamin converts the energy of G domain dimerization and GTP hydrolysis into rearrangements affecting the dynamin collar. These changes could provide the mechanochemical force needed for membrane constriction and subsequent loosening of the dynamin scaffold from the membrane, thus precipitating the membrane-remodeling events required for fission. Durieux, A.C., Prudhon, B., Guicheney, P., and Bitoun, M. (2010) J. Mol. Med. 88, 339-350.